valley firearms

It's been a long time since I really messed with carburetors. I have a 750 Race Demon and a Holly 750 dbl pumper. My 570 Street Avenger is giving up the ghost and needs rebuilt. Just wondering, can a 750 be de-tuned or re-jetted to work on my 302 until I get the 347 going? The 302 has a big comp cam. I put the Demon on the car and it revs great. It idles funny and pukes a little black smoke when I rev it. It has 74 primary jets right now.
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DenDen

You need velocity with the air running through the carb to draw the fuel out and atomize it properly.The 750 allows too much air to run thru slowly.too slow to do the job.The black smoke indicates a too rich situation caused by the gas simply falling into the manifold without being atomized.

67mustang302

You can de tune it to run, but it won't run well, 750 is WAY too much for a 302 unless you're turning 8,000rpm. No matter what you do to it, throttle response and mileage, and overall power, won't be that great, but it'll get you around. Sounds like it's way too rich, which isn't a suprise at all, you'll have to jet it down, but again, it won't run that well

iamdiffrnt

The way I understand it, too small will work ok, but power output will be limited by venturi size. Too large is bad, not enough flow through the venturi to mix/draw fuel correctly (as was stated above).

As always I'm no expert, and would appreciate being corrected.

67mustang302

Basically speaking, yeah, that's how it works. Too small will run well, just limits top end power(depending on hwo restrictive the carb is vs the engine's needs) and too big may just suck all the way around

highhilleer

It would probably be easier, and maybe cheaper, to rebuild the 570 carb than to detune one of the 750. As a bonus it will also run better.

mikethebike

I'm not trying to be a S/A but why not make a 2-barrell out of it by disconneting the secondarys and only using a single fuel line to the front? I did that once on a 600 V/S that had a cracked fuel bowl and it worked well until I got the money to buy a parts carb.
(you do all kinds of inventive stuff when your'e young and an e-3 in the Navy)

DenDen

If it is a vac sec 750,the secondaries would probably never open anyway on the 302.
Its the lower rpm range that is affected by the primary size.I think a 750 race demon is even too big for a 347.

valley firearms

Thanks guys. I put the Demon 750 and Holley 750 on Craigslist. I will look for a 600 or 650.
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67mustang302

To give you an idea, I have the 570 Street Avenger on my 302 in the 67. It SCREAMS, runs like a scalded cat, and develops plenty of top end, it keeps pulling HARD as the rpm climbs, feels peakier than I thought it would, and it still has torque up the wazoo and drives great on the street. You'd prolly be best off rebuilding the 570